adjective
chance, random
Synonyms: accidental, adventitious, by chance, contingent, erratic, extemporaneous, extempore, fluky, fortuitous, impromptu, improvised, impulsive, incidental, infrequent, irregular, occasional, odd, offhand, serendipitous, spontaneous, uncertain, unexpected, unforeseen, unintentional, unplanned, unpremeditated
Antonyms: deliberate, painstaking, planned, premeditated
adjective
nonchalant, relaxed in manner
Synonyms: aloof, apathetic, blasé, breezy, cool, cursory, detached, down home, easygoing, folksy, homey, incurious, indifferent, informal, insouciant, lackadaisical, laid-back, loose, low-pressure, mellow, offhand, perfunctory, pococurante, purposeless, remote, unconcerned, unfussy, uninterested, withdrawn
Antonyms: formal, serious